Pre-processing in openfoam, mesh generation. OpenFOAM kurs 2013 Håkan Nilsson Olivier Petit

Transcription

1 Pre-processing in openfoam, mesh generation. OpenFOAM kurs 2013 Håkan Nilsson Olivier Petit

2 Different ways of creating the mesh. Outline Using SnappyHexMesh, an OpenFOAM mesh generation tool. Using blockmesh, m4 and python. Importing the mesh from external software.

3 A tutorial for snappyhexmesh. SnappyHexMesh generates a 3D mesh from a.stl file. ( triangulated surface geometry) For this tutorial, a simplified pump geometry is chosen. For more simplicity in computations, the symmetry of the geometry is used, and only one quarter of the pump is meshed.

4 A tutorial for snappyhexmesh. Download the tutorial from Håkan s webpage. Source OpenFOAM 2.2.x with alias OF22x or export FOAM_INST_DIR=/chalmers/sw/unsup64/OpenFOAM;. $FOAM_INST_DIR/OpenFOAM-2.2.x/etc/bashrcHani To check if the right OpenFOAM was called: which SimpleFoam It should point to simplefoam in OpenFOAM-2.2.x In the tutorial case, you should find: 1. The.stl file located in constant/trisurface. 2. A dictionnary called snappyhexmeshdict in system/.

5 A tutorial for snappyhexmesh. Requirement for snappyhexmesh to work: snappyhexmeshdict in /system/ Geometry data in constant/trisurface Hexahedral base mesh (decomposed if running in parallel) decomposepardic file in /system/ All system dictionaries (e.g. controldict, fvschems, fvsolutions)

6 A tutorial for snappyhexmesh. 5 simple steps: Create base mesh Refine base mesh Rmove unused cells Snap mesh to surface Add layers

7 snappyhexmesh: step 1 Creation of a grid surrounding the stl surface. Characteristics of the grid: The aspect ratio of the grid cells should be around 1. More than one cell in the z direction. At least on cell s edge should intersect the surface. There can not be empty patches, it is a 3D mesher.

8 SnappyHexMesh: step 2 Refine the base mesh. Start the splitting from locationinmesh feature. This edge must be inside the region to be meshed and must no coincide with a cell face. Splitting the cells around the surface according to : refinementsurfaces { file.stl { level (2 2); // default (min max) refinement for surface } }

9 Snappyhexmesh: step 3 Remove unused cells. Keep the side of the mesh defined by locationinmesh. Remove all cells that have above 50% of their volumes in the meshed region. This first step is saved into the time folder 1. The region refined is specified by: inside: inside the volume region. outside: outside the volume region. distance: according to distance to the surface. The region is defined first as geometry.

10 Snappyhexmesh: step 4 Snapping to surface. The steps to snap to surface are: 1. Snap the vertices onto the STL surface. 2. Solve for relaxation of the internal mesh. 3. Iterate using the snapcontrols in SnappyHexMeshDict. This second step is saved into the time folder 2.

11 Snappyhexmesh: step 5 Boundary layer addition. The boundary are applied on patches, not on surface!! Mesh projection back from the surface using a specified thickness normal to the surface. Solve for relaxation of the internal mesh with the latest projected boundary vertices. Check if validation criteria are validated. If the validation criteria can be satisfied, insert mesh layers. This last step is saved into the time folder 3.

12 Snappyhexmesh: pros and cons. Possibilities of multiple refinements that make it very robust. It runs in parallel. Need of a good quality STL surface, with more than one region/patch. The feature line is now available. SurfaceFeatureExtract A Boundary Layer mesh is not easy to obtain, it requires experience and trial and error method.

13 SurfaceFeatureExtract Need a dictionary in system/ called ExtractFeatureDict. In the test case, the executable is surfacefeatureextract. Create a *.emesh in constant/trisurface, and a new folder in constant/ called extendedfeatureedgemesh if the option writeobj is selected in surfaceextractfeaturedict. In the snappyhexmeshdict dictionary, specify in features: { } file "pump_simplified.emesh"; level 3; // level of refinement

14 SurfaceFeatureExtract

15 EXTERNAL LINKS FOR SNAPPYHEXMESH exmesh.php AndrewJacksonSlidesOFW7.pdf

16 blockmesh/m4 m4: allow a parametrization of the case, easy to change. Let us take an example. Simple geometry: a pipe. The m4 file is found in m4_python/mesh/2d.m4. a4t a3t Front quad a1t a2t z y x a1b a4b Back quad a2b a3b

17 blockmesh/m4: pros and cons. Very easy way to create an simple geometry. The parametrization with m4 allows to create different geometries from the same m4 file. Not enough precision in the meshing parameters. Easy to go wrong on the orientation of the faces and blocks.

18 Python Python is a general-purpose, high-level programming language. It emphasizes code readabilty. Like other dynamic languages, Python is often used as a scripting language. Commonly coupled with OpenFOAM, very useful to execute, analyse, manipulate parameters/simulations in OpenFOAM. Most commonly used python library in OpenFOAM is pyfoam.

19 Using Python to create a test case Main script:./chooseshape.py Needs 2 arguments. To know which one:./chooseshape.py h arg1 is (2D, 3D, symmetry), arg2 is (rectangle, cylinder) Call an other script in pythonscript folder: geometrysetup.py The script generates the chosen geometry, changes the m4 file, and do blockmesh

20 Using Python Many different useful applications pyfoam is a very useful compilation of library. Interesting tutorial by Eric Paterson, n_scripting_for_gluing_cfd_applications A_Case_Study_Demonstrating_Automati on_of_grid_generation%2c_parameter_variation%2cflow_simulation%2c_analysis %2C_and_Plotting.html. It can also be found at post-processing example for turbomachinery can be found at tor#post-processing_using_python

21 Import the mesh: pros and cons Need of an other software to create the mesh. Few converters: fluentmeshtofoam, fluent3dmeshtofoam for Gambit mesh types. startofoam for STAR-CD mesh types. ideastofoam for I-DEAS mesh types cfx4tofoam for CFX mesh types. CGNSToFoam for CGNS files (can import more than meshes), developped by users.